1. Field of the Invention
The present invention relates to driving circuits and driving methods of liquid crystal display devices, and in particular to the polarity inversion of voltages applied to pixels in active matrix liquid crystal display devices.
2. Description of the Related Art
Active matrix liquid crystal display devices provided with TFTs (thin film transistors) as switching elements have been known for several years. Such liquid crystal display devices are provided with a liquid crystal panel which includes two insulating substrates that are arranged opposite one another. On one substrate of the liquid crystal panel, scanning signal lines and video signal lines are arranged in a lattice, and TFTs are arranged near the intersections of the scanning signal lines and the video signal lines. Each of the TFTs has a drain electrode, a gate electrode branching off from the scanning signal lines, and a source electrode branching off from the video signal lines. The drain electrodes are connected to pixel electrodes that are arranged in a matrix on the substrate for forming an image. Also, the substrate on the other side of the liquid crystal panel is provided with an opposing electrode for applying a voltage between the pixel electrodes and the opposing electrode, across the liquid crystal layer. The individual pixels are formed by the pixel electrodes, the opposing electrode and the liquid crystal layer. It should be noted that, for the sake of convenience, regions forming single pixels are referred to as “pixel formation portions”. Moreover, a voltage is applied to the pixel formation portions based on a video signal that the source electrodes of the TFTs receive from the video signal lines when the gate electrodes of the TFTs receive an active scanning signal from the scanning signal lines. Thus, the liquid crystal is driven, and the desired image is displayed on the screen.
Now, the liquid crystal has the property of degrading when a DC voltage is applied to it continuously. Therefore, an AC voltage is applied to the liquid crystal layer in the liquid crystal display device. This application of the AC voltage to the liquid crystal layer is realized by inverting the polarity of the voltage applied to each of the pixel formation portions at every single frame period, that is, by inverting at every single frame period the polarity of the voltage of the source electrode (video signal voltage) when taking the voltage of the opposing electrode as the reference. As technologies for realizing this, a driving method known as line inversion driving and a driving method known as dot inversion driving are known. It should be noted that in the following, the voltage applied to the pixel formation portions is referred to as “pixel voltage”.
In line inversion driving, the polarity of the pixel voltage is inverted at every single frame period and at every predetermined number of signal scanning lines. For example, a driving method in which the polarity of the pixel voltage is inverted at every single frame period and at every two scanning signal lines is referred to as “2-line inversion driving”. On the other hand, in dot inversion driving, the polarity of the pixel voltage is inverted at every single frame, and also the polarities of pixels that are adjacent in the horizontal direction are inverted in a single frame period. Driving method in which the polarity of the pixel voltage is inverted at every predetermined number of scanning signal lines can also be applied to dot inversion driving. For example, dot inversion driving in which the polarity of the pixel voltage is inverted at every two scanning signal lines is referred to as “2-line-dot inversion driving”.
FIG. 12 is a polarity diagram illustrating the change of the polarities of the pixel voltages in 1-line inversion driving and in 1-line-dot inversion driving. FIG. 13 is a polarity diagram showing the change of the polarities of the pixel voltages in 2-line inversion driving and in 2-line-dot inversion driving. FIGS. 12 and 13 show the polarities of the pixel voltages that are applied, for each frame period, to the pixel formation portions at the intersection between the scanning signal lines from the first row to the fourth row and the video signal line of the first column. “GL1 to GL4” denotes the scanning signal lines. “Nr. 1 to Nr. 16” denotes the frame periods. “+” and “−” indicate the pixel voltage polarity. As shown in FIGS. 12 and 13, the polarity of the pixel voltage of each of the pixel formation portions is inverted at every single frame period. It should be noted that the difference between line inversion driving and dot inversion driving lies in whether, within one frame period, there is a polarity inversion of the pixel voltage among pixels that are adjacent in the horizontal direction on the display screen. Consequently, taking note of the individual pixel formation portions, for line inversion driving and dot inversion driving alike, the polarity of the pixel voltage at every frame period changes in the same manner.
With the above-described 1-line inversion driving, flickering can be perceived when white and gray are displayed alternately at every single scanning signal line, for example. The reason for this is that the polarities of the pixel voltages of all pixel formation portions of the scanning signal lines displaying gray become the same, and the flicker components are not averaged. Moreover, also in 2-line inversion driving, when white and gray are displayed alternately at every two scanning lines, flickering can be perceived for the same reason as in the case of 1-line inversion driving.
In order to solve this problem, JP 2002-149117A proposes a liquid crystal display device that switches between 1-line inversion driving and 2-line inversion driving at every predetermined number of frame periods. FIG. 14 is a polarity diagram showing the change of the polarities of the pixel voltages in this liquid crystal display device. As shown in FIG. 14, 1-line inversion driving is performed in the first to fourth frame periods, and 2-line inversion driving is performed in the fifth to eighth frame periods. Then, the same polarity change pattern as the change of polarities of pixel voltages in the first to eighth frame periods (in the following, the change of polarities of the pixel voltages in a plurality of frame periods is referred to as “polarity change pattern”) is repeated from the ninth frame period onward. In accordance with this driving method, even if white and gray are displayed at every predetermined number of scanning signal lines, the polarities of the pixel voltages of all pixel formation portions in the scanning signal lines displaying gray will not be the same. Thus, the flickering components can be averaged, and flickering can be suppressed.
However, even with a driving method as described above, the polarities of the pixel voltages of the pixel formation portions change in a regular fashion. Therefore, there are image patterns known as “killer patterns”, in which the polarity change patterns themselves can be perceived as flickering. Thus, the degradiation of the display quality could not be prevented.